1. Field of the Invention
The invention relates to a ground primary type carrying apparatus driven by a linear motor, especially for speed control during acceleration and deceleration.
2. Description of Related Art
A carrying apparatus driven by a linear motor as a driving source, which makes a carrying vehicle move along a predetermined track, is generally driven because of thrust. This thrust is provided by an interaction between an electric current and a magnetic field, which acts upon the place between a secondary conductive plate and stators. The plate is fixed to the carrying vehicle and supports the carrying vehicle by a magnetic action force. An electro-magnet, which is in a non-contact state with a predetermined gap between itself and the track, provides the magnetic action force. The stators are provided at predetermined intervals at positions opposite to the secondary conductive plate along the track of the carrying vehicle.
In such a conventional carrying apparatus driven by a linear motor, however, the weight of the carrying vehicle changes depending on its load. The change of the gap due to the change of the weight of the carrying vehicle is stably controlled by providing, at the carrying vehicle, an electric current detecting unit. This unit is for detecting an electric current flowing through a coil of the electro-magnet. A gap sensor is also provided for detecting the gap. Thus, the electric current flowing through the coil is controlled so as to have the constant length of the gap being detected by the gap sensor.
In the case where the carrying vehicle is heavy, however, response is late for acceleration and deceleration. A long distance and time is required for predetermined acceleration and deceleration. When acceleration is insufficient, there is a problem in that the carrying vehicle's hang-up occurs because the carrying vehicle energized by a stator can't reach the next stator to energize the carrying vehicle because of running resistance, such as air resistance which is given to the carrying vehicle. When deceleration is insufficient, there is another problem in that the carrying vehicle goes too far away from the predetermined stop position.
In the case where the carrying vehicle is lighter, response at its acceleration and deceleration is so speedy that it leads to the sharp speed change at its acceleration and deceleration, resulting in a problem that shock is given to the load. There is also a problem in that the speed is too reduced at its deceleration, causing the carrying vehicle's hang-up between stators in the same way as for acceleration as previously described.
As may be clear from the above description, in the conventional carrying apparatus driven by a linear motor, it has been difficult to control the running motion of the carrying apparatus satisfactorily because response at acceleration and deceleration of the carrying vehicle changes depending on its load.
In order to solve these problems, there is a method to calculate acceleration from the speed change of the carrying vehicle and to control the acceleration to be constant, by which stabilized response at acceleration and deceleration of the carrying vehicle can be obtained.
In addition, in Japanese Patent Application Laid-Open No. 62-281703 (1987), a carrying apparatus driven by a linear motor is proposed. This apparatus makes response at its acceleration and deceleration constant by providing at its track side a gap sensor for detecting the gap length between the carrying vehicle and the track in order to measure the gap length which is converted to the weight of the carrying vehicle. Further, the driving force is adjusted so that the absolute value of the driving force of the linear motor which drives the carrying vehicle according to the increment of the weight thereof can be increased.
The aforementioned method for controlling acceleration to be constant requires a speed detector with high discrimination. Such a speed detector, however, receives an influence of noise easily and, in addition, the cost of equipment of it is expensive.
There is a problem that a control unit for carrying out compound control at high speed is required, which causes the apparatus to be expensive.
Furthermore, in the latter carrying apparatus driven by a linear motor which calculates the weight of the carrying vehicle on the basis of the gap length between the carrying vehicle and the track and which adjusts the driving force on the basis of aforesaid weight of the carrying vehicle, there is a problem that many more gap sensors must be provided at the track side thereof. This results in a problem that the cost of equipment of the apparatus becomes expensive.